The applications for the liquid crystal displays have expanded as lightweight and thin displays have become alternatives to conventional CRT displays. However, liquid crystal displays have disadvantages with respect to CRT displays, such as a small viewing angle and poor motion display characteristics. One method for solving the problems of viewing angle and motion display characteristics is to use a liquid crystal cell (.pi. cell) in which a liquid crystal layer, sandwiched between two glass substrates, assumes, what is referred to as a bend alignment. A bend alignment is characterized, as shown in FIG. 6, in that liquid crystal adjacent the glass substrates 2 has an alignment angle substantially horizontal to the glass substrates, and an angle vertical to the glass substrate surfaces toward the center of the liquid crystal layer. The alignment angle gradually continuously changes to horizontal at the opposed glass substrate surfaces as the distance from the center of the liquid crystal layer increases. There is no twisted structure over the whole liquid crystal layer. The liquid crystal display using such a liquid crystal cell is disclosed in Japanese Published Unexamined Patent Application No. 61-116329. In the same publication, an example utilizing the .pi. cell as a liquid crystal display is disclosed. Further, U.S. Pat. No. 5,410,422 of Philip J, Bos discloses an idea of combining an optical compensation film with such .pi. cell to obtain a wide viewing angle. Furthermore, in recent years, as Uchida et al. discloses in Japanese Published Unexamined Patent Application No. 7-84254, the optical characteristics of the optical compensation film used with the cell has been examined, and the feasibility of a wider viewing angle is discussed.
The .pi. cell (a liquid crystal cell using a liquid crystal layer having a bend alignment) not only provides a wide viewing angle characteristic, but also a fast response time. This provides the possibility for dramatically increasing the motion display characteristics of liquid crystal cells.
A liquid crystal cell to be operated as the .pi. cell does not take a bend alignment in an initial state in which no electric field or the like is applied to it. In such a state, it assumes a homogeneous alignment. If the transition from a homogeneous state to a bend alignment cannot be performed at high speed, good motion display characteristics of the bend alignment state cannot be fully exploited. None of the above described references discusses this point.
The transition of the liquid crystal from a homogeneous state to the bend alignment state is performed through an intermediate state called a splay alignment. When a voltage is applied to a liquid crystal layer and the applied voltage is equal to or higher than a certain threshold voltage V.sub.CR, the electric elastic energy of the bend alignment state is lower than the electric elastic energy of the splay alignment state. Accordingly, a bend alignment state is stable for a voltage equal to or higher than the threshold voltage V.sub.CR, but a transition between the splay alignment state and the bend alignment state does not generally occur because of an energy barrier that exists between them. The generation of a transition nucleus is required as a condition for the occurrence of the transition. However since the energy for such a generation itself is high, it is difficult for the transition to proceed.
Generally, when power is turned on, a voltage several times greater than V.sub.CR is applied to accelerate transition. The process of alignment transition of liquid crystal is described using FIGS. 3 to 6. As shown in FIG. 3, initially, a liquid crystal layer 1 is in a homogeneous state. In this figure, the liquid crystal layer 1 is sandwiched between glass substrates 2 and 3. If a large voltage is applied, a splay alignment as shown in FIG. 4 is exhibited. The splay alignment state shown in FIG. 4 is observed during a very short period of time immediately after the power is turned on. It has a symmetrical structure 4 in the center of the cell. Such a centralized symmetrical splay structure is called a first-stage splay alignment. The first-stage splay alignment bears a striking resemblance to the bend alignment shown in FIG. 6 in that it is symmetrical in the center of the cell, and accordingly a transition from a first stage splay alignment to a bend alignment can be extremely fast. However, the first-stage splay alignment state is very unstable, and, as shown in FIG. 5, the symmetrical structure 4 quickly moves to the vicinity of the upper or lower glass substrate 2 or 3 to form a more stable second-stage splay alignment. The speed of the transition from a second-stage splay alignment to a bend alignment is relatively slow.
It is reported by Oku et al. in the Proceedings of General Conference of the Electronic Information Communication Society, 1996, p. 88, that the transition speed for the direct transition from the first-state splay alignment to a bend alignment is an order of magnitude faster than the transition from a second-stage splay alignment to a bend alignment. Accordingly, it would be preferable if the bend alignment state can be obtained directly from the first-stage splay alignment. However, a first-stage splay alignment's stage life is short difficult to maintain, and up until now, no method for accomplishing such a transition is known.
It was found that the transition from the first to the second stage splay alignment gradually proceeds from the outer periphery of the electrode. It is thought that, since the liquid crystal cells of a matrix-driven liquid crystal display are partitioned by vertical and lateral electrodes, a nonuniform electric field that exists in the cells contributes to this behavior. For instance, in the commonly used driving method called a H-COM inversion, because of difference in the voltage polarity between adjacent pixels or differences between the electrode potential and the pixel electrode potential, distortion which occurs in the electric field vertical to the cell surface, induces a transition to the second-stage splay alignment. Since the second-stage splay alignment having occurred in the peripheral portion of a pixel is essentially stable, it causes the first-stage splay alignment existing in the center of the pixel to transition to the second-stage splay alignment. Accordingly, it is not often that a first-stage splay alignment directly transitions to a bend alignment. In most cases, it transitions to the bend state through a second-stage splay alignment. Thus, more time is consumed for the whole pixel to reach the bend state.
Further, even if a certain pixel directly transitions to a bend alignment, the adjacent pixels do not generally transition to a bend alignment in a chain reaction. The reason for this is that, since the vertical electric field in the vicinity of electrodes existing around the pixels is smaller than the other portion, the stability of the bend alignment state is small in terms of electric elastic energy, and thus the driving force for transition to a bend alignment is also small. Accordingly, the region in the vicinity of electrodes acts as a barrier to bend alignment expansion from a pixel to a pixel.
As described above, in the conventional driving method, in a cell structure having a partitioned pixel structure, the life of usable first-stage splay alignments is short and most of first-stage splay alignments transition to second-state splay alignments. Thus, a sufficiently fast transition speed to a bend alignment cannot be obtained. Further, there is a possibility that bend alignment transition between pixels is prevented by small regions of a small vertical electric field existing between pixels, or the transition to the bend alignment is incomplete or pixels with no bend alignment occur.